[PATCH 2/4]: powerpc/cell spidernet low watermark patch.

[Benjamin Herrenschmidt]

On Fri, 2006-08-11 at 12:08 -0500, Linas Vepstas wrote:
> 
> Implement basic low-watermark support for the transmit queue.
> 
> The basic idea of a low-watermark interrupt is as follows.
> The device driver queues up a bunch of packets for the hardware
> to transmit, and then kicks he hardware to get it started.
> As the hardware drains the queue of pending, untransmitted 
> packets, the device driver will want to know when the queue
> is almost empty, so that it can queue some more packets.
> 
> This is accomplished by setting the DESCR_TXDESFLG flag in
> one of the packets. When the hardware sees this flag, it will 
> interrupt the device driver. Because this flag is on a fixed
> packet, rather than at  fixed location in the queue, the
> code below needs to move the flag as more packets are
> queued up. This implementation attempts to keep te flag 
> at about 3/4's of the way into the queue.
> 
> This patch boosts driver performance from about 
> 300-400Mbps for 1500 byte packets, to about 710-740Mbps.

Sounds good (without actually looking at the code though :), that was a
long required improvement to that driver. Also, we should probably look
into using NAPI polling for tx completion queue as well, no ?

Cheers,
Ben.